The present invention relates to a magnetic recording medium having excellent electromagnetic transfer characteristics and high durability and comprising at least one or more magnetic layers with ferromagnetic powder and a binder dispersed therein and a lower layer containing at least one of either magnetic powder or non-magnetic powder and a binder dispersed therein, and the lower layer and the magnetic layers are placed on a non-magnetic support.
The magnetic recording medium is widely used in such applications as recording tape, video tape, floppy disk, etc. The magnetic recording medium comprises a magnetic layer having ferromagnetic powder and a binder dispersed in it, and the magnetic layer is laminated on a non-magnetic support.
The magnetic recording medium must be at high level in such characteristics as electromagnetic transfer characteristics, running durability, and running performance. Specifically, an audio tape for reproducing musical sound must have an ability to reproduce original sound with higher quality. A video tape must have high electromagnetic transfer characteristics such as high ability to reproduce original image or picture.
Together with the requirements to have high electromagnetic transfer characteristics, the magnetic recording medium must have good running durability as described above. To improve durability and electromagnetic transfer characteristics, the binder, serving as one of major components of the magnetic layer, plays an important role. When vinyl chloride resin, cellulose resin, urethane resin, acrylic resin, etc. as used in the past are adopted, it is disadvantageous in that abrasive resistance of the magnetic layer is low, and running components of the magnetic tape are contaminated.
Contamination of the magnetic head causes deterioration of electromagnetic transfer characteristics. In particular, in the device for high density recording, number of revolutions of the magnetic head is high. In a digital video tape recorder, number of revolutions of the magnetic head is as high as 9,600 rpm, and this is much higher compared with 1,800 rpm of an analog video tape recorder for household use, and 5,000 rpm of a video tape recorder for business use. Sliding speed between the magnetic recording medium and the magnetic head is getting increasingly higher, and the magnetic head and the thin film head are designed in more and more compact size, and there are strong demands to minimize and to improve the problems of magnetic head contamination caused by components of the magnetic recording medium.
To solve the above problems, there have been proposed several methods to increase hardness of the magnetic layer using hard binder.
For example, JP-A-6-96437 describes a magnetic recording medium, in which a polyester polyurethane having urethane group concentration of 2.5 mmol/g or more and a polyvinyl acetal resin are used. JP-A-7-50010 describes a substance having high urethane group concentration in polyurethane, which contains polyol of 0 to 5 mol %. In both cases, dispersion property is not satisfactory, and none of these inventions describes that an effect is attained by a combination with a specific type of ferromagnetic powder.
JP-A-3-88119 (EP 415,708) discloses a magnetic recording medium, which contains an aliphatic urethane in a lower layer and aromatic urethane in an upper layer. Also, JP-A-6-259746 describes a urethane, which comprises a polyester polyol using branched glycol, cycloapliphatic glycol, and aromatic or cycloaliphatic dibasic acid. JP-A-6-76265 discloses a polyurethane, which comprises a polyester diol using branched polyhydric alcohol, and JP-A-6-314424 describes a polyurethane, which comprises a polyester polyol using 3-methyl-1,5-pentanediol and an aliphatic dicarboxylic acid. However, the polyurethane disclosed in each of these inventions has low glass transition temperature (Tg). This means that the entire coating film or surface of the coating film are softened. When it is used in a video tape, the coating film tends to be fluidized due to sliding and friction with the rotating head, and running durability is decreased. When it is stored under high temperature conditions, the surface of the coating film is softened. This increases friction coefficient and leads to poor running condition.
A magnetic recording medium, which comprises yttrium-containing ferromagnetic powder is disclosed in each of JP-A-7-210856 (U.S. Pat. No. 5,580,399) and JP-A-7-272253. JP-A-7-272253 describes the use of a polyurethane, but it simply gives description on the use of a binder commonly in use, and it is disadvantageous in that it is difficult to disperse ferromagnetic powder because magnetic energy among the ferromagnetic powder particles is high.
It is an object of the present invention to provide a magnetic recording medium, which has ferromagnetic powder with high dispersion property, exhibits excellent electromagnetic transfer characteristics, has coating film surface with high strength and shows very good running durability.